A Novel Ternary Catalyst PW4@MOF-808@SBA-15 for Deep Extraction Oxidation Desulfurization of Model Diesel

In this work, a novel heterogeneous catalyst consisting of peroxophosphotungstate, microporous MOF-808, and mesoporous SBA-15 was synthesized, characterized, and used to remove sulfides from model fuel. The prepared material, PW4@MOF-808@SBA-15, exhibits excellent catalytic activity with a desulfurization efficiency of 99.8% in 60 min for multicomponent simulated fuel, and the desulfurization rate can reach more than 90% after ten consecutive cycles. The excellent catalytic activity and reusability are attributed to the hierarchically porous hybrid material MOF-808@SBA-15, which can effectively encapsulate PW4 and provide a site for the oxidation of sulfides.


Introduction
With the continuous growth in the global demand for energy, the environmental problems caused by the massive use of traditional petrochemical energy have gradually become prominent [1,2].Among them, problems such as air pollution and acid rain caused by the sulfur oxides emitted from fuel combustion are particularly prominent [3,4].In view of the environmental issues caused by the combustion of sulfur-containing compounds in liquid fuels, strict environmental laws have been enacted around the world to limit the sulfur content in fuels, such as below 10 ppm for diesel [5,6].There are many kinds of sulfides in fuel, which can be divided into two categories: active sulfur species (elemental sulfur, hydrogen sulfide, mercaptans) and inactive sulfur species (aromatic sulfides and heterocyclic sulfides) [7,8].Among them, the removal of refractory organic sulfur compounds such as dibenzothiophene and its derivatives is challenging.Therefore, it is necessary to find efficient and economical desulfurization methods.Traditional hydrodesulfurization technology (HDS) is a method of removing sulfides from fuel, which converts organic sulfur compounds to hydrogen sulfide and hydrocarbons by reacting with hydrogen under the action of a catalyst [9][10][11].However, HDS can only effectively remove the active sulfides and part of the inactive sulfides, and reaction conditions are harsh, which led to a burst of research on alternative methods such as extraction oxidation desulfurization technology (EODS) [12][13][14].Pure extractive desulfurization (EDS) is a method to extract sulfide from fuel oil by using the difference in the solubility of sulfide in specific solvents and fuel oil.Oxidative desulfurization (ODS) can convert the organic sulfide in fuel into a more polar substance, and then the more polar substance is further separated from the fuel by extraction or adsorption to achieve the purpose of reducing the sulfur content.EODS is a combination of extraction and oxidation processes, in which sulfides are first extracted into the extraction phase and then oxidized to sulfoxides and/or sulfones in the presence of an oxidant and a catalyst.The process continues until a low-sulfur or even sulfur-free fuel is obtained [15,16].EODS can be carried out with an oxidant and without any catalyst but is generally less effective and requires extremely harsh reaction conditions such as high temperature and pressure.Therefore, highly active and environmentally friendly catalysts are needed for the efficient removal of refractory sulfides through EODS.Common oxidants include O 2 , O 3 , H 2 O 2, and tert-butyl peroxide, among which H 2 O 2 is the most widely used oxidant during the EODS process because it is cheap, easily available, and environmentally friendly, and has a high active oxygen content [17][18][19][20].Several metal-containing substances, including metal oxide, polyoxometalates (POMs), metal complexes, and metal-organic frameworks, have been applied to EODS [21][22][23].In particular, polyoxometalates have become attractive candidates for EODS in recent years due to their simple synthesis, stable structure, and adjustable structure and properties [24].Polyoxometalates (POMs) with peroxy bonds exhibit higher catalytic activity in EODS than traditional Keggin-type polyoxometalates [25,26].This is due to the higher electrophilicity of some of the oxygen atoms in peroxopolyoxometalate, which can effectively improve the EODS efficiency.Salete et al. [26] prepared Keggin-type phosphotungstates modified with different cations ([BPy] 3 PW 12 , [BMIM] 3 PW 12, and [HDPy] 3 PW 12 ), which effectively removed several refractory sulfides from fuel using [BMIM]PF 6 as the extraction agent.Meanwhile, the formation of peroxy complexes (PW x O y ), which play a key role in the catalytic reaction, was verified through 31 P NMR.In view of the above conclusions, the authors prepared peroxophosphotungstate (nBu 4 N) 3 {PO 4 [WO(O 2 ) 2 ] 4 } for fuel desulfurization, and the results showed that the catalyst could achieve a high desulfurization rate under more economical and energy-saving conditions [25].Shi and co-workers [27] also prepared heterogeneous catalyst SiO 2 -BisILs [(PW 12 O 40 ) 3− ] assembled from peroxophosphotungstate and ionic liquid brush via EODS, and the prepared material showed excellent catalytic activity for dibenzothiophene (DBT).Although POMs show superior activity in EODS, they cannot be separated from fuel due to their high solubility and cannot be reused continuously.Encapsulating POMs in solid materials with a large specific surface area and suitable pore structure is an effective way to solve the above shortcomings.
Metal-organic frameworks (MOFs), a class of ordered crystalline compounds, have become promising contemporary materials due to their tunable chemical functionalities, high specific surface area, and pore volume [28].The development of new hybrid systems, which can combine the advantages of each component, has been attracting a large number of researchers.Some researchers have tried to combine MOFs with different materials, such as materials with two-dimensional structures (carbon nanotubes, graphene, and boron nitride) or materials with three-dimensional pore structures (MCM-41 and SBA-15), and so on.These MOF-based composites not only combine the advantages of the two materials but also show unique properties.Tapas Kumar Maji and Anindita Chakraborty [29] prepared a hybrid Mg-MOF-74@SBA-15 material through immobilizing Mg-MOF-74 nanocrystals into the mesopores of SBA-15, which has remarkable CO 2 adsorption capacity at room temperature due to its unique pore structure.Dirk E. De Vos and co-workers [30] obtained a hybrid catalyst, (Zr)UiO-66(NH 2 )/SBA-15, by means of "solid-state" crystallization, i.e., the selective growth of (Zr)UiO-66(NH 2 ) nanocrystals in mesoporous SBA-15, and the authors noted that this hybrid material, (Zr)UiO-66(NH 2 )/SBA-15, has higher catalytic activity and mechanical stability than pure MOFs.
MOF-808, as one of the {Zr 6 O 8 }-cluster-based MOFs, has attracted extensive attention due to its high hydrophilic porous surface, good biocompatibility, large cavity, and pore volume.However, the poor stability of MOF-808 has become the main factor restricting its large-scale commercial application.Mesoporous molecular sieve SBA-15 has attracted much attention from researchers due to its highly ordered mesoporous structure, good thermal stability, and high mechanical strength [31].The hierarchically porous composite constructed by MOF material and mesoporous molecular sieve SBA-15 has a rich chemical environment and stable mechanical properties, which can avoid the disadvantages of MOF materials and provide more possibilities for the application of MOFs [29,30,32].
In this work, a more robust heterogeneous catalyst was constructed by combining peroxophosphotungstate [(C 4 H 9 ) 4 N] 3 {PO 4 [WO(O 2 ) 2 ] 4 }, marked as (PW 4 ), and hierarchically porous composite MOF-808@SBA-15 through a simple hydrothermal strategy, which was used for the removal of sulfides from model fuel.The structure of the target product was analyzed by various characterization methods, and then the desulfurization performance was studied using the multicomponent model fuel.

Characterization
The crystal structures of the prepared materials were analyzed by powder XRD, as depicted in Figure 1.The obtained pattern for PW 4 shows several characteristic peaks at 7-30 • .For pure MOF-808, many strong peaks are shown at 2θ values of 4.3, 8.3, and 8.7 • , which were assigned to the characteristic planes (111), (311), and (222).Tri-peaks were observed at low angles in the SBA-15 pattern, which were labeled as p6mm hexagonally symmetrical (100), (110), and (200) reflections, confirming the successful preparation of SBA-15.After incorporating PW 4 , no significant change was found in the characteristic peak, indicating that PW 4 was uniformly dispersed in the SBA-15 channel.For PW 4 @MOF-808@SBA-15, the presence of the same signals from MOF-808 confirmed the successful incorporation of MOF-808 into SBA-15, and the absence of signals from PW 4 indicated uniform encapsulation of PW 4 .Furthermore, the low-angle peaks in the two composites (PW 4 @SBA-15 and PW 4 @MOF-808@SBA-15) were similar to those of SBA-15, indicating that the ordered mesoporous pores corresponding to SBA-15 were not destroyed.
In this work, a more robust heterogeneous catalyst was constructed by combining peroxophosphotungstate [(C4H9)4N]3{PO4[WO(O2)2]4}, marked as (PW4), and hierarchically porous composite MOF-808@SBA-15 through a simple hydrothermal strategy, which was used for the removal of sulfides from model fuel.The structure of the target product was analyzed by various characterization methods, and then the desulfurization performance was studied using the multicomponent model fuel.

Characterization
The crystal structures of the prepared materials were analyzed by powder XRD, as depicted in Figure 1.The obtained pattern for PW4 shows several characteristic peaks at 7-30°.For pure MOF-808, many strong peaks are shown at 2θ values of 4.3, 8.3, and 8.7°, which were assigned to the characteristic planes (111), (311), and (222).Tri-peaks were observed at low angles in the SBA-15 pattern, which were labeled as p6mm hexagonally symmetrical (100), (110), and (200) reflections, confirming the successful preparation of SBA-15.After incorporating PW4, no significant change was found in the characteristic peak, indicating that PW4 was uniformly dispersed in the SBA-15 channel.For PW4@MOF-808@SBA-15, the presence of the same signals from MOF-808 confirmed the successful incorporation of MOF-808 into SBA-15, and the absence of signals from PW4 indicated uniform encapsulation of PW4.Furthermore, the low-angle peaks in the two composites (PW4@SBA-15 and PW4@MOF-808@SBA-15) were similar to those of SBA-15, indicating that the ordered mesoporous pores corresponding to SBA-15 were not destroyed.The IR spectra of the raw materials (PW4, MOF-808, and SBA-15) and two composites (PW4@SBA-15 and PW4@MOF-808@SBA-15) were recorded and are displayed in Figure 2.For PW4, several intense bands were observed at the range of 849-1083 cm −1 , which is attributed to the Venturello structure anion {PO4[WO(O2)2]4} 3 .For MOF-808, the strong peak at 652 cm −1 is attributed to the stretching vibration of Zr-O, and two absorption bands are shown at 1618 and 1571 cm −1 , corresponding to COO -asymmetric stretching.In the case of SBA-15, the strong absorption peaks at 1031, 813, and 460 cm −1 are attributed to the asymmetric and symmetrical tensile vibration and bending patterns of the Si-O-Si frame.The FT-IR spectrum of PW4@SBA-15 shows not only the characteristic peaks belonging to SBA-15 but also some characteristic peaks belonging to PW4.In the FT-IR spectrum of PW4@MOF-808@SBA-15, the same absorptions as those of SBA-15 and MOF-808 were observed, with a slight shift, indicating the successful immobilization of MOF-808, and the peaks attributed to PW4 were not witnessed, indicating the homogeneous dispersion of PW4 within the pore structure.The IR spectra of the raw materials (PW 4 , MOF-808, and SBA-15) and two composites (PW 4 @SBA-15 and PW 4 @MOF-808@SBA-15) were recorded and are displayed in Figure 2.For PW 4 , several intense bands were observed at the range of 849-1083 cm −1 , which is attributed to the Venturello structure anion {PO 4 [WO(O 2 ) 2 ] 4 } 3 .For MOF-808, the strong peak at 652 cm −1 is attributed to the stretching vibration of Zr-O, and two absorption bands are shown at 1618 and 1571 cm −1 , corresponding to COO -asymmetric stretching.In the case of SBA-15, the strong absorption peaks at 1031, 813, and 460 cm −1 are attributed to the asymmetric and symmetrical tensile vibration and bending patterns of the Si-O-Si frame.The FT-IR spectrum of PW 4 @SBA-15 shows not only the characteristic peaks belonging to SBA-15 but also some characteristic peaks belonging to PW 4 .In the FT-IR spectrum of PW 4 @MOF-808@SBA-15, the same absorptions as those of SBA-15 and MOF-808 were observed, with a slight shift, indicating the successful immobilization of MOF-808, and the peaks attributed to PW 4 were not witnessed, indicating the homogeneous dispersion of PW 4 within the pore structure.The trends in the thermogravimetric curves of SBA-15 and PW4@SBA-15 were similar except for the weight loss rate, which verified the presence of the active component PW4 in PW4@SBA-15 (Figure 3).The thermogravimetric curve for PW4@MOF-808@SBA-15 shows a similar profile to that of MOF-808.After the weight loss of water and solvent, two significant weight losses occurred between 258 °C and 590 °C, which correspond to the removal of BTC and Zr clusters.The porous properties of pure SBA-15, PW4@SBA-15, and PW4@MOF-808@SBA-15 were determined through N2 adsorption-desorption experiments, and the test results are shown in Figure 4 and Table 1.The isotherm of the SBA-15 sample was a typical type IV with an H1-type hysteresis loop, indicating the presence of mesoporous pores.And the pore size distribution of the original SBA-15 was narrow, mainly in the range of 4.1-4.6 nm.The isotherm type of composite PW4@SBA-15 is was same as that of the original SBA-15, indicating that the original pore structure was preserved after loading PW4.The isotherm of PW4@MOF-808@SBA-15 exhibited a combination of type I (characteristic of microporous materials) and type IV (characteristic of mesoporous materials).That is, composite PW4@MOF-808@SBA-15 showed the presence of additional micropores compared to the mesoporous pores of the original SBA-15.Compared with pure SBA-15, the specific surface area and empty volume of PW4@SBA-15 decreased, caused by the occupation of PW4 in the channels.And the specific surface area and empty volume of PW4@MOF-808@SBA-15 had more obvious decreases, indicating that MOF-808 nanocrystals were partially filled in the channels.Although the SEM images (Figure 5) show the presence of few MOF-808 nanocrystals on the surface of SBA-15, the reduced The trends in the thermogravimetric curves of SBA-15 and PW 4 @SBA-15 were similar except for the weight loss rate, which verified the presence of the active component PW 4 in PW 4 @SBA-15 (Figure 3).The thermogravimetric curve for PW 4 @MOF-808@SBA-15 shows a similar profile to that of MOF-808.After the weight loss of water and solvent, two significant weight losses occurred between 258 • C and 590 • C, which correspond to the removal of BTC and Zr clusters.The trends in the thermogravimetric curves of SBA-15 and PW4@SBA-15 were similar except for the weight loss rate, which verified the presence of the active component PW4 in PW4@SBA-15 (Figure 3).The thermogravimetric curve for PW4@MOF-808@SBA-15 shows a similar profile to that of MOF-808.After the weight loss of water and solvent, two significant weight losses occurred between 258 °C and 590 °C, which correspond to the removal of BTC and Zr clusters.The porous properties of pure SBA-15, PW4@SBA-15, and PW4@MOF-808@SBA-15 were determined through N2 adsorption-desorption experiments, and the test results are shown in Figure 4 and Table 1.The isotherm of the SBA-15 sample was a typical type IV with an H1-type hysteresis loop, indicating the presence of mesoporous pores.And the pore size distribution of the original SBA-15 was narrow, mainly in the range of 4.1-4.6 nm.The isotherm type of composite PW4@SBA-15 is was same as that of the original SBA-15, indicating that the original pore structure was preserved after loading PW4.The isotherm of PW4@MOF-808@SBA-15 exhibited a combination of type I (characteristic of microporous materials) and type IV (characteristic of mesoporous materials).That is, composite PW4@MOF-808@SBA-15 showed the presence of additional micropores compared to the mesoporous pores of the original SBA-15.Compared with pure SBA-15, the specific surface area and empty volume of PW4@SBA-15 decreased, caused by the occupation of PW4 in the channels.And the specific surface area and empty volume of PW4@MOF-808@SBA-15 had more obvious decreases, indicating that MOF-808 nanocrystals were partially filled in the channels.Although the SEM images (Figure 5) show the presence of few MOF-808 nanocrystals on the surface of SBA-15, the reduced The porous properties of pure SBA-15, PW 4 @SBA-15, and PW 4 @MOF-808@SBA-15 were determined through N 2 adsorption-desorption experiments, and the test results are shown in Figure 4 and Table 1.The isotherm of the SBA-15 sample was a typical type IV with an H1-type hysteresis loop, indicating the presence of mesoporous pores.And the pore size distribution of the original SBA-15 was narrow, mainly in the range of 4.1-4.6 nm.The isotherm type of composite PW 4 @SBA-15 is was same as that of the original SBA-15, indicating that the original pore structure was preserved after loading PW 4 .The isotherm of PW 4 @MOF-808@SBA-15 exhibited a combination of type I (characteristic of microporous materials) and type IV (characteristic of mesoporous materials).That is, composite PW 4 @MOF-808@SBA-15 showed the presence of additional micropores compared to the mesoporous pores of the original SBA-15.Compared with pure SBA-15, the specific surface area and empty volume of PW 4 @SBA-15 decreased, caused by the occupation of PW 4 in the channels.And the specific surface area and empty volume of PW 4 @MOF-808@SBA-15 had more obvious decreases, indicating that MOF-808 nanocrystals were partially filled in the channels.Although the SEM images (Figure 5) show the presence of few MOF-808 nanocrystals on the surface of SBA-15, the reduced adsorption amount and partial filling of the mesopores of SBA-15 confirmed that the channels of SBA-15 could serve as sites for the growth of MOF-808 nanoparticles [29].In addition, it was possible to confirm that the channels of SBA-15 were occupied by comparing the size distribution plot.
Molecules 2024, 29, x FOR PEER REVIEW 5 of 15 adsorption amount and partial filling of the mesopores of SBA-15 confirmed that the channels of SBA-15 could serve as sites for the growth of MOF-808 nanoparticles [29].In addition, it was possible to confirm that the channels of SBA-15 were occupied by comparing the size distribution plot.Table 1.BET surface area, average pore size, and total pore volume of SBA-15, PW4@SBA-15, and PW4@MOF-808@SBA-15.The morphologies SBA-15, PW4@SBA-15, and PW4@MOF-808@SBA-15 were observed from SEM images (Figure 5).The SEM images show that SBA-15 was an irregular mass, and the morphology of PW4@SBA-15 had no significant change after the introduction of PW4.Although the appearance of PW4@MOF-808@SBA-15 was similar to that of pure SBA-15, the formation of MOF-808 on the surface of SBA-15 could be clearly seen.The chemical composition of PW4@MOF-808@SBA-15 was further evaluated using the EDS technique.The detection of characteristic elements P and W indicated the successful introduction of the active component PW4.The morphologies SBA-15, PW 4 @SBA-15, and PW 4 @MOF-808@SBA-15 were observed from SEM images (Figure 5).The SEM images show that SBA-15 was an irregular mass, and the morphology of PW 4 @SBA-15 had no significant change after the introduction of PW 4 .Although the appearance of PW 4 @MOF-808@SBA-15 was similar to that of pure SBA-15, the formation of MOF-808 on the surface of SBA-15 could be clearly seen.The chemical composition of PW 4 @MOF-808@SBA-15 was further evaluated using the EDS technique.The detection of characteristic elements P and W indicated the successful introduction of the active component PW 4 .

Sample SBET (m²/g) V (cm
The TEM image of PW 4 @SBA-15 shows clear areas and darker areas corresponding to empty holes and hole walls, which are similar to those of pure SBA-15 (Figure 6).At the same time, scattered black spots were observed in the pores, indicating that PW 4 was evenly distributed in the pores of SBA-15.The TEM image of PW 4 @MOF-808@SBA-15 shows that the black spots were denser and larger, which may have been because the pore channels were occupied by MOF-808, and the octahedral MOF-808 could be seen on the edge of the sample, which is also consistent with the SEM results.Elemental mapping was used to confirm the homogeneous distribution of guests in the SBA-15 sample.For PW 4 @SBA-15, the uniform distribution of P and W elements attributed to PW 4 indicated that the active components were evenly distributed in the pores (Figure 7).For PW 4 @MOF-808@SBA-15, the P and W elements attributed to the PW 4 and Zr elements attributed to MOF-808 were uniformly distributed, which further indicated that the active component and MOF-808 existed in the SBA-15 channels (Figure 8).The TEM image of PW4@SBA-15 shows clear areas and darker areas corresponding to empty holes and hole walls, which are similar to those of pure SBA-15 (Figure 6).At the same time, scattered black spots were observed in the pores, indicating that PW4 was evenly distributed in the pores of SBA-15.The TEM image of PW4@MOF-808@SBA-15 shows that the black spots were denser and larger, which may have been because the pore channels were occupied by MOF-808, and the octahedral MOF-808 could be seen on the edge of the sample, which is also consistent with the SEM results.Elemental mapping was used to confirm the homogeneous distribution of guests in the SBA-15 sample.For PW4@SBA-15, the uniform distribution of P and W elements attributed to PW4 indicated that the active components were evenly distributed in the pores (Figure 7).For PW4@MOF-808@SBA-15, the P and W elements attributed to the PW4 and Zr elements attributed to MOF-808 were uniformly distributed, which further indicated that the active component and MOF-808 existed in the SBA-15 channels (Figure 8).The composition and chemical environment of PW4@SBA-15 and PW4@M 808@SBA-15 were further elucidated by XPS (Figure 9).The survey scan of composites, PW4@SBA-15 and PW4@MOF-808@SBA-15, revealed the presence of W 1s, O 1s, N 1s, and P 2p elements, while Zr 3d was detected in PW4@MOF-808@SB The high-resolution spectra of W 4f for PW4@SBA-15 and PW4@MOF-808@SBA-1 shown in Figure 9b.For PW4@SBA-15, the binding energies of 37.5 (W 4f5/2) and 35 (W 4f7/2) were assigned to the W(VI) oxidation state.For PW4@MOF-808@SBA-15    The composition and chemical environment of PW4@SBA-15 and PW4@M 808@SBA-15 were further elucidated by XPS (Figure 9).The survey scan of composites, PW4@SBA-15 and PW4@MOF-808@SBA-15, revealed the presence of W 1s, O 1s, N 1s, and P 2p elements, while Zr 3d was detected in PW4@MOF-808@SB The high-resolution spectra of W 4f for PW4@SBA-15 and PW4@MOF-808@SBA-1 shown in Figure 9b.For PW4@SBA-15, the binding energies of 37.5 (W 4f5/2) and 35 (W 4f7/2) were assigned to the W(VI) oxidation state.For PW4@MOF-808@SBA-15 The composition and chemical environment of PW 4 @SBA-15 and PW 4 @MOF-808@SBA-15 were further elucidated by XPS (Figure 9).The survey scan of two composites, PW 4 @SBA-15 and PW 4 @MOF-808@SBA-15, revealed the presence of W 4f, C 1s, O 1s, N 1s, and P 2p elements, while Zr 3d was detected in PW 4 @MOF-808@SBA-15.The high-resolution spectra of W 4f for PW 4 @SBA-15 and PW 4 @MOF-808@SBA-15 are shown in Figure 9b.For PW 4 @SBA-15, the binding energies of 37.5 (W 4f 5/2 ) and 35.4 eV (W 4f 7/2 ) were assigned to the W(VI) oxidation state.For PW 4 @MOF-808@SBA-15, the negative chemical shift (ca.0.16 eV) indicated the chemical interaction between PW 4 and MOF-808 [33].In addition, a peak of W(CO)x appeared at 31.4 eV, which further proved that the active ingredient PW 4 may have been covalently linked to MOF-808 via the -COOH functional group on the organic linker [34].Additionally, the high resolution of the Zr 3d spectrum of PW 4 @MOF-808@SBA-15 showed two peaks at 185.1 and 182.7 eV, which were assigned to Zr 3d 5/2 and Zr 3d 3/2 .
MOF-808 [33].In addition, a peak of W(CO)x appeared at 31.4 eV, which further proved that the active ingredient PW4 may have been covalently linked to MOF-808 via the -COOH functional group on the organic linker [34].Additionally, the high resolution of the Zr 3d spectrum of PW4@MOF-808@SBA-15 showed two peaks at 185.1 and 182.7 eV, which were assigned to Zr 3d5/2 and Zr 3d3/2.

EODS Performance of Different Catalysts
The desulfurization of the simulated diesel containing dibenzothiophene (DBT), 4methyldibenzothiophene (4-MDBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT), which are the most common refractory sulfur compounds in liquid fuels, was carried out using MeCN as the extraction agent, H2O2 as the oxidant, and different materials as the catalyst.In the typical desulfurization process, a biphasic system consisting of 0.75 mL of simulated diesel and equal volumes MeCN and 3 umol PW4 or composite containing 3 umol PW4 were strongly stirred for 10 min under 70 ℃ to activate the initial extraction of sulfur compounds from the model diesel to the extraction solvent.Then, H2O2 (0.21 mmol, H2O2/S molar ratio = 6) was poured into the system to start the oxidation stage.Periodically, the upper fuel phase was sampled to analyze the variation in sulfide concentration.
Pure PW4, as a homogeneous catalyst, demonstrated superior desulfurization performance, removing 99.2% of the sulfur in the simulated fuel within 40 min (Figure 10a).Although PW4 showed excellent catalytic activity in the catalytic oxidation of sulfides, it was difficult to recover from the reaction medium, which restricts its development in industrial applications.The heterogeneous catalyst prepared by filling PW4 in the SBA-15 channel for fuel desulfurization effectively improved the recyclability of the catalyst.The kinetic profile of the heterogeneous catalyst PW4@SBA-15 was similar to that of the homogeneous catalyst PW4, but only 98.9% of the sulfides was removed within an extended 100 min.Pure MOF-808 displayed moderate desulfurization activity with a sulfur removal rate of 77.1% in 100 min.Compared with pure MOF-808 and PW4@SBA-15, the ternary catalyst PW4@MOF-808@SBA-15 could remove sulfur compounds from fuel with higher efficiency, which was attributed to the uniform dispersion of PW4 molecules as a single active site and the synergistic action between PW4 and MOF-808.To verify that the active component PW4 was confined to the pore material, a leaching experiment was performed over the PW4@MOF-808@SBA-15 catalyst (Figure 10b).The results showed that the removal rate of sulfide remained basically unchanged after the catalyst filtration, indicating that the active substance was stably confined.Pure PW 4 , as a homogeneous catalyst, demonstrated superior desulfurization performance, removing 99.2% of the sulfur in the simulated fuel within 40 min (Figure 10a).Although PW 4 showed excellent catalytic activity in the catalytic oxidation of sulfides, it was difficult to recover from the reaction medium, which restricts its development in industrial applications.The heterogeneous catalyst prepared by filling PW 4 in the SBA-15 channel for fuel desulfurization effectively improved the recyclability of the catalyst.The kinetic profile of the heterogeneous catalyst PW 4 @SBA-15 was similar to that of the homogeneous catalyst PW 4 , but only 98.9% of the sulfides was removed within an extended 100 min.Pure MOF-808 displayed moderate desulfurization activity with a sulfur removal rate of 77.1% in 100 min.Compared with pure MOF-808 and PW 4 @SBA-15, the ternary catalyst PW 4 @MOF-808@SBA-15 could remove sulfur compounds from fuel with higher efficiency, which was attributed to the uniform dispersion of PW 4 molecules as a single active site and the synergistic action between PW 4 and MOF-808.To verify that the active component PW 4 was confined to the pore material, a leaching experiment was performed over the PW 4 @MOF-808@SBA-15 catalyst (Figure 10b).The results showed that the removal rate of sulfide remained basically unchanged after the catalyst filtration, indicating that the active substance was stably confined.
Considering the excellent catalytic activity of PW 4 @MOF-808@SBA-15 for model diesel, the ternary catalyst PW 4 @MOF-808@SBA-15 was used to desulfurize commercial diesel with a primary sulfur concentration of 1271 ppm.After treatment under the same conditions, the sulfur concentration of the commercial diesel was reduced to 105 ppm, corresponding to a 91.7% desulfurization rate.The desulfurization efficiency for commercial diesel was lower than that for simulated diesel, which may have been due to the more complex types of sulfides in real diesel.Considering the excellent catalytic activity of PW4@MOF-808@SBA-15 for model diesel, the ternary catalyst PW4@MOF-808@SBA-15 was used to desulfurize commercial diesel with a primary sulfur concentration of 1271 ppm.After treatment under the same conditions, the sulfur concentration of the commercial diesel was reduced to 105 ppm, corresponding to a 91.7% desulfurization rate.The desulfurization efficiency for commercial diesel was lower than that for simulated diesel, which may have been due to the more complex types of sulfides in real diesel.

Reusability
The reusability of both heterogeneous catalysts PW4@SBA-15 and PW4@MOF-808@SBA-15 was measured in order to assess their practical application potential (Figure 11).The solid catalyst remained in the MeCN extraction layer during the reaction process.After a single test, the reused catalyst was centrifuged, washed, and dried, and then added to a new desulfurization system containing fresh simulated diesel, extractant, and oxidizer for the next ECODS cycle.The results showed that the desulfurization activity of PW4@SBA-15 decreased significantly after six consecutive cycles with a sulfide removal rate of 89.5%, which may have been due to the loss of PW4 adsorbed in the SBA-15 channel with the progression of the test.The reusability of the ternary catalyst PW4@MOF-808@SBA-15 was significantly better than that of composite PW4@SBA-15, which may have been due to the presence of MOF-808 inhibiting the loss of active component PW4.

Reusability
The reusability of both heterogeneous catalysts PW 4 @SBA-15 and PW 4 @MOF-808@SBA-15 was measured in order to assess their practical application potential (Figure 11).The solid catalyst remained in the MeCN extraction layer during the reaction process.After a single test, the reused catalyst was centrifuged, washed, and dried, and then added to a new desulfurization system containing fresh simulated diesel, extractant, and oxidizer for the next ECODS cycle.The results showed that the desulfurization activity of PW 4 @SBA-15 decreased significantly after six consecutive cycles with a sulfide removal rate of 89.5%, which may have been due to the loss of PW 4 adsorbed in the SBA-15 channel with the progression of the test.The reusability of the ternary catalyst PW 4 @MOF-808@SBA-15 was significantly better than that of composite PW 4 @SBA-15, which may have been due to the presence of MOF-808 inhibiting the loss of active component PW 4 .Considering the excellent catalytic activity of PW4@MOF-808@SBA-15 for model diesel, the ternary catalyst PW4@MOF-808@SBA-15 was used to desulfurize commercial diesel with a primary sulfur concentration of 1271 ppm.After treatment under the same conditions, the sulfur concentration of the commercial diesel was reduced to 105 ppm, corresponding to a 91.7% desulfurization rate.The desulfurization efficiency for commercial diesel was lower than that for simulated diesel, which may have been due to the more complex types of sulfides in real diesel.

Reusability
The reusability of both heterogeneous catalysts PW4@SBA-15 and PW4@MOF-808@SBA-15 was measured in order to assess their practical application potential (Figure 11).The solid catalyst remained in the MeCN extraction layer during the reaction process.After a single test, the reused catalyst was centrifuged, washed, and dried, and then added to a new desulfurization system containing fresh simulated diesel, extractant, and oxidizer for the next ECODS cycle.The results showed that the desulfurization activity of PW4@SBA-15 decreased significantly after six consecutive cycles with a sulfide removal rate of 89.5%, which may have been due to the loss of PW4 adsorbed in the SBA-15 channel with the progression of the test.The reusability of the ternary catalyst PW4@MOF-808@SBA-15 was significantly better than that of composite PW4@SBA-15, which may have been due to the presence of MOF-808 inhibiting the loss of active component PW4.

Comparison with Other Catalysts
A comparison of the removal of organic sulfur substrates from model fuel catalyzed by the ternary catalyst PW 4 @MOF-808@SBA-15 prepared in this work and POM-based catalysts reported in the literature is given in Table 2 and Figure 12.Compared with similar catalysts (PMo 12 @MOF-808@SBA-15, SRL-POM@MOF-199@MCM-41, and POM-MOF@Fibercloth), the catalyst PW 4 @MOF-808@SBA-15 prepared in this work can remove a variety of sulfides from fuel in a shorter time, which is mainly due to the high activity of the active component PW 4 .Compared with simple composites, the catalyst PW 4 @MOF-808@SBA-15 has the advantage of high catalytic activity against a variety of sulfides under mild conditions.mild conditions.

Possible Mechanism
On the basis of the reported studies, a possible mechanism for the removal of organic sulfides from model fuel by EODS technology is proposed.The EODS system consists of an upper fuel phase and a lower extraction phase, and the catalyst PW4@MOF-808@SBA-15 exists in the lower extraction phase.At the initial stage of the reaction, the sulfides in

Possible Mechanism
On the basis of the reported studies, a possible mechanism for the removal of organic sulfides from model fuel by EODS technology is proposed.The EODS system consists of an upper fuel phase and a lower extraction phase, and the catalyst PW 4 @MOF-808@SBA-15 exists in the lower extraction phase.At the initial stage of the reaction, the sulfides in the fuel phase were first extracted into the lower extractant, and the initial extraction rate was in the range of 53.6-60.8%(as shown in Figure 10).Then, the addition of the oxidizing agent H 2 O 2 initiated the catalytic oxidation reaction of the sulfides in the extraction phase.While the sulfides were oxidized to large polar sulfones, more sulfides in the upper fuel phase were continuously transferred to the extraction layer until a low-sulfur or sulfur-free fuel was obtained.During the catalytic oxidation stage, PW 4 in the PW 4 @MOF-808@SBA-15 catalyst combined with H 2 O 2 to form an active species, which is a key intermediate in the oxidation reaction of POM-based catalysts with H 2 O 2 as an oxidizing agent [42].Under the action of the active species, the sulfides were oxidized to sulfoxides, then further oxidized to sulfones (Figure 13) [43].
phase were continuously transferred to the extraction layer until a low-sulfur or sulfurfree fuel was obtained.During the catalytic oxidation stage, PW4 in the PW4@MOF-808@SBA-15 catalyst combined with H2O2 to form an active species, which is a key intermediate in the oxidation reaction of POM-based catalysts with H2O2 as an oxidizing agent [42].Under the action of the active species, the sulfides were oxidized to sulfoxides, then further oxidized to sulfones (Figure 13) [43].

Synthesis of PW4@SBA-15
Peroxophosphotungstate (Bu4N)3{PO4[WO(O2)2]4}, abbreviated as PW4, was obtained according to our previous work [44].SBA-15 was obtained by the following experimental method: 1.5 g of pluronic P123 was poured into 56.3mL of 2.4 M HCl aqueous under stirring at 40 °C, and then 3.3 g of TEOS was added to the above solution.The mixed solution was stirred for 24 h at 40 °C and then heated at 100 °C for 24 h.After cooling, the SBA-15 sample was obtained by calcining the isolated white solid for 5 h at 550 °C.C, and then 3.3 g of TEOS was added to the above solution.The mixed solution was stirred for 24 h at 40 • C and then heated at 100 • C for 24 h.After cooling, the SBA-15 sample was obtained by calcining the isolated white solid for 5 h at 550 • C. PW 4 @SBA-15 was prepared by the impregnation method.Then, 1.0 g of SBA-15 was added to 5 mL of acetonitrile solution dissolved with 0.50 g of PW 4 under magnetic stirring.After 24 h, the solid product, PW 4 @SBA-15, was collected and dried.
3.2.2.Synthesis of PW 4 @MOF-808@SBA-15 PW 4 @MOF-808@SBA-15 was obtained through the solvothermal method [35,36], as follows: 0.50 g of SBA-15, 0.50 g of PW 4 , 0.51 g of ZrCl 4, and 0.16 g of H 3 BTC were poured into 25 mL of DMF and treated with ultrasound for 30 min.Then, the mixture was transferred to Teflon autoclaves and heated at 130 • C for 24 h.Finally, the solid was collected after cooling, washed with DMF and EtOH three times, and dried at 60 • C overnight; the yield was 63.5%.

Catalyst Characterization
The functional group information of different samples was acquired through a Fouriertransform infrared (FT-IR) spectrometer (EQUINOX 55) using the KBr pellet method in the wavenumber range of 4000-400 cm −1 (Bruker, Ettlingen, Germany).The crystal structures of the prepared materials were acquired by powder X-ray diffraction (XRD) on a Bruker D8 Advance (Bruker, Ettlingen, Germany) and Shimadzu XRD-7000S (Shimadzu, Tokyo, Japan) at room temperature.The thermal stability of the prepared materials was analyzed on a NETZSCH STA 449A thermal analyzer (Netzsch-GeräTebau GmbH, Selb, Germany) from 30 to 1000 • C with a heating rate of 5 • C/min under a nitrogen atmosphere.The physical structure parameters of the porous materials were collected by a N 2 adsorption-desorption apparatus on a JW-BK 200 at 77 K (CASIO, Tokyo, Japan), and all samples were degassed at 120 • C for 6 h before the test.The morphology and element composition of the samples were examined by a scanning electron microscope (SEM, SU8010, Hitachi, Tokyo, Japan) with electron energy-dispersive spectroscopy (EDX) at 3 kV.The element distribution of the samples was further examined by transmission electron microscopy (TEM) on a Talos F200X instrument (Thermo Fisher Scientific, Waltham, MA, USA).Before the test, about 5 mg of the powder sample was fully dispersed in 1 mL of ethanol under ultrasound and then dropped on the surface of the copper mesh.The surface chemical composition and electronic states of the materials were analyzed by X-ray photoelectron spectroscopy (XPS) on PHI-Veroprobe 5000 III spectrometer (ULVAC-PHI, Maozaki, Japan) with an Al Kα X-ray source.The sulfide concentration during the reaction was periodically measured by a gas chromatograph (Agilent 7890, Agilent Technologies, Santa Clara, CA, USA) equipped with an HP-5 type (30 m × 0.25 mm) column; the temperatures of the inlet, detector, and oven were 150 • C, 250 • C, and 150 • C, respectively.

Evaluation of EODS Efficiency
A given amount of DBT, 4-MDBT, and 4,6-DMDBT was completely dissolved in noctane to obtain a simulated diesel with a total sulfur content of 1500 ppm.EODS tests were executed in a closed glass container filled with simulated oil, acetonitrile as the extractant, and catalyst under continuous stirring.More precisely, 3 µmol PW 4 or composite containing 3 µmol PW 4 , 0.75 mL of model diesel, and an equal volume of acetonitrile were added to the reactor in sequence and stirred for 10 min under 70 • C to achieve extraction equilibrium.Then, the oxidation process was initiated by adding 0.21 mmol of hydrogen peroxide to the mixture.During the whole process, the upper diesel phase was periodically sampled, and the sulfide content was quantitatively analyzed by the internal standard method for gas chromatography.

Catalyst Recovery
After the reaction, the mixture was centrifuged for 10 min to recover the solid catalyst.Then, the catalyst was fully washed with octane and acetonitrile and dried overnight at 60 • C. The recovered catalyst was added to the fresh EODS system to re-evaluate its desulfurization performance.Several consecutive tests were carried out to assess its reusability.

Conclusions
A novel ternary catalyst, PW 4 @MOF-808@SBA-15, was successfully prepared through the facile solvothermal method in the presence of mesoporous SBA-15 and active-center PW 4 for the deep desulfurization of fuel.The characterization test results showed that the small MOF-808 nanocrystals grew in the pores and surfaces of SBA-15 to form a complex, MOF-808@SBA-15, and PW 4 was encapsulated within the pores of MOF-808@SBA-15.The ternary catalyst PW 4 @MOF-808@SBA-15 showed high desulfurization activity and stability in multicomponent model fuel.This is mainly due to three reasons: the high dispersion of the active-center PW 4 unit, the presence of mesoporous channels promoting the diffusion of reactants and products, and the mesoporous silica carrier improving the stability of the material.Although this ternary catalyst, PW 4 @MOF-808@SBA-15, had an excellent desulfurization effect with a simulated fuel, its application in real fuel needs to be further improved.This new type of ternary composite will open new directions in materials science, as the hybridization of different components will yield more new properties.

Figure 2 .
Figure 2. FT-IR spectra of the obtained materials.

Figure 3 .
Figure 3. Thermogravimetric curves of the obtained materials.

Figure 2 .
Figure 2. FT-IR spectra of the obtained materials.

Figure 2 .
Figure 2. FT-IR spectra of the obtained materials.

Figure 3 .
Figure 3. Thermogravimetric curves of the obtained materials.

Figure 3 .
Figure 3. Thermogravimetric curves of the obtained materials.

2. 2 .
Catalytic Performance 2.2.1.EODS Performance of Different Catalysts The desulfurization of the simulated diesel containing dibenzothiophene (DBT), 4methyldibenzothiophene (4-MDBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT), which are the most common refractory sulfur compounds in liquid fuels, was carried out using MeCN as the extraction agent, H 2 O 2 as the oxidant, and different materials as the catalyst.In the typical desulfurization process, a biphasic system consisting of 0.75 mL of simulated diesel and equal volumes MeCN and 3 umol PW 4 or composite containing 3 umol PW 4 were strongly stirred for 10 min under 70 °C to activate the initial extraction of sulfur compounds from the model diesel to the extraction solvent.Then, H 2 O 2 (0.21 mmol, H 2 O 2 /S molar ratio = 6) was poured into the system to start the oxidation stage.Periodically, the upper fuel phase was sampled to analyze the variation in sulfide concentration.

Figure 10 .
Figure 10.Desulfurization of the multicomponent model diesel catalyzed by different materials (a) and leaching experiments of the catalyst PW 4 @MOF-808@SBA-15 (b).

Table 2 .
Comparison of catalytic systems for oxidative desulfurization.
* The model fuel contained several sulfides at the same time.

Table 2 .
Comparison of catalytic systems for oxidative desulfurization.
* The model fuel contained several sulfides at the same time.